1. Technical Field
The present invention generally relates to the field of volume control and more particularly, is directed to television signal volume control having optimum stereo separation.
2. Related Art
A conventional television stereo audio baseband spectrum is illustrated in FIG. 1. This spectrum is representative of BTSC stereo (Broadcast Television System Committee) commonly known as MTS stereo (Multichannel Television Sound). The MTS audio spectrum comprises main channel 110 and stereo subchannel 111. The MTS spectrum is compatible with both mono and stereo television. A mono television will receive and play only main channel 110. A stereo television will receive main channel 110 and stereo subchannel 111 and combine them to separately play left and right stereo sound channels. Main channel 110 carries the sum of the left and right channels (L+R). Stereo subchannel 111 carries the difference of the right from the left channel (L-R). Stereo subchannel 111 is compressed by DBX.RTM. signal compressors and expanders available from BSR North America, Ltd., as used in commercially available MTS encoders and decoders. The compression of (L-R) stereo subchannel 111 occurs on the MTS stereo signal when modulated for transmission to a television. At the television, an MTS decoder will decompress the (L-R) stereo subchannel 111. After decompression the MTS decoder combines (L+R) main channel 110 with (L-R) stereo subchannel 111 to yield the separate left and right channels of stereo sound. Main channel 110 and stereo subchannel 111 are combined according to equations 1 and 2: EQU (L+R)+(L-R)=2L (1.) EQU (L+R)-(L-R)=2R (2.)
After decoding by the MTS decoder, these separate left and right channels can then be adjusted in amplitude for television volume control.
A problem has been discovered when one desires to adjust the volume of an encoded stereo television signal prior to expansion. A stereo television signal prior to decoding contains a compressed stereo channel such as stereo subchannel 111. When an audio baseband signal similar to FIG. 1 is attenuated to adjust volume, the compressed subchannel 111 is attenuated as equally as noncompressed channel 110. However, after expansion of subchannel 111 by a stereo decoder, subchannel 111 can have nearly double the attenuation as main channel 110. Poor stereo separation thus results when the doubly attenuated (L-R) subchannel 111 is combined with (L+R) main channel 110.
FIG. 2(a) illustrates compression and expansion without attenuation while FIG. 2(b) illustrates the double attenuation effects of compression and expansion with attenuation. In FIGS. 2(a) and 2(b), subchannel 111 is illustrated prior to compression at 112 compressed at 113 and expanded at 114. FIG. 2(b) illustrates attenuation of the compressed signal at 113 creating double the attenuation of expanded signal 114. Equations 3 and 4 demonstrate how attenuation A of main channel 110 and double attenuation 2A of stereo subchannel 111 provide for poor stereo separation: EQU A(L+R)+2A(L-R) (3.) EQU A(L+R)-2A(L-R)=A(3R-L) (4.)
Compressing a channel for transmission reduces noise. The compression used in MTS stereo compresses stereo subchannel 111 as illustrated in FIGS. 2(a) and 2(b). Compressed signal 113 is illustrated raised above noise level 120 to reduce noise upon transmission.
Often in the home, audio volume is adjusted prior to tuning and/or stereo decoding in a television. Set top cable TV converters 210 often perform audio volume control in addition to channel tuning prior to reception by television 211, as illustrated in FIG. 3. Usually the set top converter 210 will have a remote volume and channel control in addition to television 211 having a remote control. Additionally, it is possible that video cassette recorder (VCR) 212 may perform audio volume control prior to reception by television 211. In such an instance a home viewer would have volume control on any of set top converter 210, VCR 212 or television 211.
The discovered stereo separation problem can arise in the above configuration of FIG. 3 when set top converter 210 adjusts audio volume by attenuating the non-compressed (L+R) main channel and the compressed (L-R) stereo subchannel. When the (L-R) stereo subchannel is expanded and combined with the non-compressed (L+R) main channel, poor stereo separation occurs.
Set top converters known in the art are only capable of adjusting volume when their power is on. This can pose a problem should the volume be preset at a high level. When the converter is turned on a disturbing high level sound can result on the speakers associated with a television. Furthermore, prior to turning on the set top converter, a user had no way of determining the volume level.
A set top converter with an internal video cassette recorder (VCR) timer turns on the set top converter at a preprogrammed time and tunes to a preprogrammed channel for taping at VCR 212. Conventionally, when a set top converter is turned on, the volume is set at a previously manually set volume level. This posed a problem since use of the VCR timer would not always provide optimum stereo separation.